Hybrid electric vehicles (HEVs), whose operation is characterized by both discharge and regenerative tractive electrical loads and nontractive electrical loads, often employ batteries to provide energy boost or storage for short periods such as acceleration, engine start-up, and regenerative braking, as well as for relatively-longer-duration discharge pulses. As a result, the batteries experience very high and frequent surge currents and, therefore, must withstand thousands of deep charge/discharge cycles which can potentially impact battery life. Further, such batteries must be optimized for high-power capability, generally resulting in a substantially increased cost.
In response, the prior art teaches use of energy storage systems featuring double-layer capacitors, also known as supercapacitors or ultracapacitors, that are electrically coupled in parallel with a battery as shown in FIG. 1, to handle rapid transients while the batteries handle lengthy demand peaks and provide long-term energy storage. These ultracapacitors behave like high-power, low-capacity batteries, except that ultracapacitors store electric energy by accumulating and separating unlike-charges physically, rather than storing energy chemically in reversible chemical reactions. As such, ultracapacitors can provide high power and can accept high power during charging, even as their instantaneous voltage approaches their maximum rated voltage. Advantageously, ultracapacitors also typically feature a high cycle life and a high cycle efficiency, as compared to commercially-available chemical batteries.
Referring again to FIG. 1, with the ultracapacitor UC and the battery B connected in parallel, and assuming both a constant simulated switched load current IL (to simplify the following analysis) and a parasitic series resistance RC,RB of both the ultracapacitor and the battery, upon the closure of switch S1 at time t=0, the capacitor and battery currents iC,iB, are calculated as follows:
                              i          C                =                              I            L                    [                                                    R                B                            ⁢                                                          ⁢                              ⅇ                                                      -                    t                                    /                  τ                                                                                    R                B                            +                              R                C                                              ]                                    (        1        )                                          i          B                =                              I            L                    [                      1            -                                                            R                  B                                ⁢                                                                  ⁢                                  ⅇ                                                            -                      t                                        /                    τ                                                                                                R                  B                                +                                  R                  C                                                              ]                                    (        2        )                                          where          ⁢                                          ⁢          τ                =                              (                                          R                B                            +                              R                C                                      )                    ⁢                                          ⁢          C          ⁢                                          ⁢          is          ⁢                                          ⁢          the          ⁢                                          ⁢          circuit          ⁢                                          ⁢          time          ⁢                                          ⁢          constant                                    (        3        )            The resulting peak value ILmax for the switched load current IL at time t=0 is calculated as:
                              I                      L            ⁢                                                  ⁢            max                          =                                            V              B                                      (                                                                    R                    B                                    ⁢                                      R                    C                                                                                        R                    B                                    +                                      R                    C                                                              )                                =                                                    V                B                                            R                B                                      +                                          V                B                                            R                C                                                                        (        4        )            Thus, if the resistance RC of the ultracapacitor is significantly less than the resistance RB of the battery, the prior art's teaching of the parallel coupling of the ultracapacitor and the battery, as shown in FIG. 1, advantageously provides a higher maximum switched load current ILmax (thereby improving vehicle cold weather starting) while a correlative reduction of battery current iB pulse amplitudes reduces degradation of the battery.
By way of further example, assuming a resistance RC for the ultracapacitor of 10 mΩ at 25° C. and 11.5 mΩ at −30° C., and a resistance RB for the battery of 54 mΩ at 25° C. and 180 mΩ at −30° C., Equations (1) and (2) demonstrate that, as the ultracapacitor current iC decreases in response to longer-duration or higher-energy discharge pulses, the battery current iB will increase, particularly at relatively higher ambient temperatures:@25° C.: IC max=5.4IB max @−30° C.: IC max=15.65IB max Thus, for a ten-second vehicle start/acceleration pulse for a power-assist hybrid electric vehicle (P-HEV), as set forth in P-HEV Power & Energy Design Verification Load Profile published by the U.S. Department of Energy in the “FreedomCAR 42V Battery Test Manual” (April 2003), reproduced in FIG. 2A, Equations (1)-(3) yield the following:τ=9.3 sec@25° C.iC=29% of IL@t=10 seciB=71% of IL@t=10 secτ=27.8 sec@−30° C.iC=66% of IL@t=10 seciB=34% of IL@t=10 secThe foregoing confirms that known energy storage systems featuring a parallel-coupled ultracapacitor and battery provide much higher discharge pulses than an energy storage system featuring a battery alone.
However, the foregoing also confirms that the parallel-coupled ultracapacitor and battery results in a battery current iB portion of the switched load current IL being very high, particularly with increasing ambient temperature. And, while the ultracapacitor generally has very high immunity to high discharge and regenerative pulses such as vehicle start/acceleration pulses and regenerative braking, known chemical batteries typically do not share such a high immunity, and such shared pulses can therefore cause significant degradation of the parallel-coupled batteries due to battery cycling.